A new study says that 100 cases of West Nile virus infection -- and a dozen… (Anne Cusack / Los Angeles…)
About 100 cases of West Nile virus infection -- and a dozen deaths -- could have been prevented last year if public health officials in Dallas had kept track of the proportion of infected mosquitoes caught in traps, a new study says.
A detailed analysis of the 2012 outbreak in Dallas gave researchers the perfect opportunity to test a long-held theory that monitoring mosquitoes, rather than human cases, could predict epidemics and allow for early intervention. Keeping tabs on mosquitoes allows public health officials to estimate the rate of infection in the local mosquito population, as well as the mosquito population size; multiplying the two numbers produces a figure called the "vector index." Experts at the Centers for Disease Control and Prevention thought the vector index would make it easier to see where the virus was spreading and take early action to slow or halt that spread.
West Nile virus, which is most commonly transmitted by mosquitoes, can cause fever, encephalitis and meningitis, according to the CDC. Although most people who are infected won’t display symptoms, about 1 in 5 will develop a fever with other symptoms. Less than 1% of patients will develop a serious, sometimes fatal, neurologic illness. Currently, no medications or vaccines for the virus exist.
The virus first emerged in New York City in 1999. It then spread across the U.S, with the first outbreak in Dallas occurring in 2006. After a five-year lull, the virus came roaring back in 2012, infecting 7.3 out of every 100,000 residents and killing 19 people.
Dr. Robert Haley, an epidemiologist with UT Southwestern in Dallas, and his colleagues calculated the vector index for each week during the 2012 epidemic. Frequent monitoring is crucial, because symptoms normally take a week to appear and subsequent lab tests can take another two to three weeks to produce results. By then, the epidemic would have almost finished running its course.
“It can take three or four weeks before you know about people who are infected,” said William Reisen, an epidemiologist at UC Davis who was not part of the study. “By that time, there are that many more infected, and you’re far, far behind.”
Based on those figures, they used statistical models to determine that Dallas public health officials could have prevented 100 cases and 12 deaths had they intervened a week after the vector index hit its threshold. Instead, they took action only after several patients had been hospitalized.
Weather conditions in 2006 and 2012 created “a perfect storm” for a West Nile epidemic, Haley said. Within the 11-year period studied, both years had the fewest hard freeze days — when the temperature low measured less than 28 degrees Fahrenheit — and the warmest spring temperatures. Both years also saw drought punctuated by rainstorms “so there was always a little standing water around,” Haley explained.
Experts say that global warming trends forecast more epidemics. “Diseases that typically only occurred in the tropics are starting to creep northward,” Haley said. “West Nile is one of them.”
Merging surveillance with Census tract data, the researchers also found that, in both the 2006 and 2012 epidemics, cases tended to cluster around affluent, housing-dense areas with a higher percentage of unoccupied homes — not in low socioeconomic neighborhoods, as often occurs with other urban epidemics.